Integrated circuits are interconnected networks of electrical components fabricated on a common foundation called a substrate. The electrical components are typically fabricated on a wafer of semiconductor material that serves as a substrate. Various fabrication techniques, such as layering, doping, masking, and etching, are used to build millions of resistors, transistors, and other electrical components on the wafer. The components are then wired together, or interconnected, to define a specific electrical circuit, such as a processor or a memory device.
Fusible elements are employed in integrated circuits to permit changes in the configuration of the integrated circuits after fabrication. For example, fusible elements may be used to replace defective circuits with redundant circuits. Memory devices are typically fabricated with redundant memory cells. The redundant memory cells may be enabled with fusible elements after fabrication to replace defective memory cells found during a test of fabricated memory devices. Fusible elements are also used to customize the configuration of a generic integrated circuit after it is fabricated, or to identify an integrated circuit. One type of fusible element is a polysilicon fuse. The polysilicon fuse comprises a polysilicon conductor fabricated to conduct electrical current on an integrated circuit. A portion of the polysilicon fuse may be evaporated or opened by a laser beam to create an open circuit between terminals of the polysilicon fuse. The laser beam may be used to open selected polysilicon fuses in an integrated circuit to change its configuration. The use of polysilicon fuses is attended by several disadvantages. Polysilicon fuses must be spaced apart from each other in an integrated circuit such that when one of them is being opened by a laser beam the other polysilicon fuses are not damaged. A bank of polysilicon fuses therefore occupies a substantial area of an integrated circuit. In addition, polysilicon fuses cannot be opened once an integrated circuit is placed in an integrated circuit package, or is encapsulated in any manner.
Another type of fusible element is an antifuse. An antifuse comprises two conductive terminals separated by an insulator or a dielectric, and is fabricated as an open circuit. The antifuse is programmed by applying a high voltage across its terminals to rupture the insulator and form an electrical path between the terminals.
Antifuses have several advantages that are not available with fuses. A bank of antifuses takes up much less area of an integrated circuit because they are programmed by a voltage difference that can be supplied on wires connected to the terminals of each of the antifuses. The antifuses may be placed close together in the bank, and adjacent antifuses are not at risk when one is being programmed. Antifuses may also be programmed after an integrated circuit is placed in an integrated circuit package, or encapsulated, by applying appropriate signals to pins of the package. This is a significant advantage for several reasons. First, an integrated circuit may be tested after it is in a package, and may then be repaired by replacing defective circuits with redundant circuits by programming selected antifuses. A generic integrated circuit may be tested and placed in a package before it is configured to meet the specifications of a customer. This reduces the delay between a customer order and shipment. The use of antifuses to customize generic integrated circuits also improves the production yield for integrated circuits because the same generic integrated circuit may be produced to meet the needs of a wide variety of customers.
Despite their advantages, the use of antifuses in integrated circuits is limited by a lack of adequate circuitry to support the programming and reading of the antifuses. There exists a need for improved circuits and methods for programming and reading antifuses in integrated circuits.